As two detection methods of a surface acoustic wave sensor, there are a phase detection method (for example, refers to Patent Documents 1 to 5) and an oscillation method (for example, refers to Patent Document 6).
The surface acoustic wave sensor of the phase detection method, for example, includes a surface acoustic wave (SAW) device, an oscillation circuit, and a phase detection circuit. The SAW device, for example, includes a piezoelectric substrate, a drive electrode (IDT pair) and a reflector. The drive electrode and the reflector are arranged at a surface of the piezoelectric substrate.
A measurement principle of the surface acoustic wave sensor of the phase detection method is as the following. When an excitation signal (burst signal) corresponding to a resonant frequency of the drive electrode is transmitted from the oscillation circuit, an electrical signal is converted to a mechanical oscillation by the drive electrode. Then, a SAW is excited on the surface of the piezoelectric substrate. The SAW excited by the drive electrode is transmitted along a propagation portion on the surface of the piezoelectric substrate, and is reflected by the reflector. The reflected SAW is converted to an electrical signal by the drive electrode, and the converted electrical signal is outputted toward the phase detection circuit. In a case where the signal inputted from the SAW device to the phase detection circuit is compared to the signal directly inputted from the oscillation circuit to the phase detection circuit when the SAW is excited, a phase of the SAW outputted from the SAW device is changed by an electrical or mechanical characteristic change of the propagation portion. Therefore, a physical quantity to be measured can be detected by the phase.
The surface acoustic wave sensor of the phase detection method may be used as a gas sensor (for example, refers to Patent Documents 1, 2), a liquid density sensor (for example, refers to Patent Document 3), a liquid characteristic sensor (for example, refers to Patent Document 4), or a cell characteristic detection device (for example, refers to Patent Document 5).
In the gas sensor, a gas sensitive membrane is arranged at the propagation portion so as to sense a gas. When the gas sensitive membrane is a selective membrane, a mass effect is generated because the gas is adsorbed by the membrane. Therefore, a gas density can be detected according to a propagation characteristic change of the SAW. In the liquid characteristic sensor, a liquid is spread on the propagation portion so as to sense a liquid characteristic. When the liquid is spread on the propagation portion, because a phase and amplitude of the SAW are changed by a conductivity of the liquid, a permittivity of the liquid, a mass of the liquid, and a viscosity of the liquid, the liquid characteristic can be detected.
The surface acoustic wave sensor of the oscillation method includes the SAW device and the oscillation circuit. The physical quantity is detected according to an oscillation frequency (resonant frequency of the SAW device) change oscillated in the oscillation circuit. For example, when a strain is generated in the propagation portion of the SAW device, the resonant frequency of the SAW device is changed according to the strain generated in the propagation portion. Therefore, the strain generated in the SAW device can be measured from the resonant frequency change.
In the surface acoustic wave sensor of the oscillation method, the resonant frequency of the SAW is significantly changed according to a temperature change. Therefore, at least two SAW devices are used so as to reduce the resonant frequency change generated by the temperature change (for example, refers to Patent Document 6).
[Prior Art Documents]
[Patent Document 1] JP-A-2002-26688
[Patent Document 2] JP-A-2009-109261
[Patent Document 3] JP-A-2008-275503
[Patent Document 4] JP-A-2008-309779
[Patent Document 5] JP-A-2010-29193
[Patent Document 6] JP-A-2008-541121